1. Field of the Invention
The present invention relates generally to RMS-to-DC converters, and more particularly, to an RMS-to-DC converter that utilizes balanced squaring cells and is capable of measuring true power at microwave frequencies.
2. Description of the Related Art
RMS-to-DC converters are used to convert the RMS (root-mean-square) value of an arbitrary signal into a quasi-DC signal that represents the true power level of the signal. Numerous techniques have been devised for performing RMS-to-DC conversions. One of the most fundamental is known as the xe2x80x9cthermalxe2x80x9d method. With the thermal method, the signal is used to generate heat in a resistive dissipator. The heat is then measured, usually by establishing a temperature balance using a second dissipator. The DC input to the second dissipator then provides a measure of the RMS value of the signal. Another technique involves xe2x80x9ccomputingxe2x80x9d converters which utilize nonlinear analog signal processing. Examples of computing converters include an xe2x80x9cexplicitxe2x80x9d converter, which utilizes an amplitude squaring cell followed by a filter and then a square rooter, and an xe2x80x9cimplicitxe2x80x9d converter which utilizes an absolute value cell followed by a squarer-divider and a filter embedded in the a feedback loop.
Another type of computing converter which operates on the xe2x80x9cdifference of squaresxe2x80x9d principle. This circuit utilizes a differential input, four-quadrant multiplier and shares some of the features of both the thermal technique, and the previously described computing techniques. Like the advanced thermal techniques, it seeks to null the difference between the square of the input and the DC output. However, like the other computing converters, it utilizes nonlinear signal processing elements. All of these techniques are discussed more thoroughly in an article by Barrie Gilbert: xe2x80x9cNovel Technique For R.M.S.xe2x80x94D.C. Conversion Based On The Difference Of Squares,xe2x80x9d Electronics Letters, Apr. 17, 1975, Vol. 11, No. 8, pp. 181-182.
Although the techniques discussed above can provide an accurate measure of the true RMS value of a signal at relatively low frequencies, they do not operate well at microwave frequencies, i.e., upwards of 1 GHz. Signal measuring devices capable of operation at microwave frequencies are available, e.g., diode detectors, but they are not true RMS detectors. Instead, they are essentially xe2x80x9cenvelopexe2x80x9d detectors which respond to the amplitude of the modulation envelope of a signal (and power indirectly), rather than responding inherently to the power of a complex waveform such as a CDMA carrier and its noise-like modulation.
Accordingly, a need remains for an improved technique for measuring the true RMS value of a signal.
An RMS-to-DC converter constructed in accordance with the present invention implements the xe2x80x9cdifference-of-squaresxe2x80x9d function by utilizing two identical squaring cells operating in opposition to generate two currents. An error amplifier nulls the difference between the currents by feedback proportional to the RMS value of the signal to one of the two squaring cells.
When used in a measurement mode, one of the squaring cells receives the signal to be measured, and the output of the error amplifier, which provides a measure of the true RMS value of the input signal, is connected to the input of the second squaring cell, thereby closing the feedback loop around the second squaring cell and establishing the scaling factor.
When used in a control mode, a set-point signal is applied to the second squaring cell, and the output of the error amplifier is used to control a variable-gain device such as a power amplifier which provides the input to the first squaring cell, thereby closing the feedback loop around the first squaring cell. Since the feedback loop is always closed around one of the two squaring cells, an implicit square-root function is implemented.
An RMS-to-DC converter constructed in accordance with the present invention can also be operated as a power comparator, in which case there is no feedback connection. In this mode, the signal to be measured is applied to the first squaring cell, a threshold signal is applied to the second squaring cell, and the output from the nulling circuit swings towards one of the power supply voltages depending on whether the RMS value of the measured signal is greater or less than the threshold signal.
By implementing the squaring cells as series-connected three-transistor multi-tanh transconductance cells using a suitable integrated circuit technology, accurate square law approximation from DC up to microwave frequencies can be achieved.
By using carefully balanced squaring cells and a well-balanced error amplifier, some of the inherent approximation errors are essentially cancelled.
One embodiment uses feedback bootstrapping to equalize the common mode voltage at the inputs to the squaring cells. By equalizing the common mode voltages at the common emitter nodes of the transconductance squaring cells, the balance of the overall structure is improved.
Another embodiment implements feedforward bootstrapping using an op amp to balance the voltages at the common nodes of the transconductance squaring cells. This also serves to provide a balanced differential input drive to one of the squaring cells.
A base current compensation circuit is used in both of the squaring cells, thereby minimizing errors caused by certain DC offset voltages generated by base currents.